Extreme ultraviolet light source apparatus and method of generating extreme ultraviolet light

ABSTRACT

An extreme ultraviolet light source apparatus, which is to generate an extreme ultraviolet light by irradiating a target with a main pulse laser light after irradiating the target with a prepulse laser light, the extreme ultraviolet light source apparatus comprises: a prepulse laser light source generating a pre-plasma by irradiating the target with the prepulse laser light while a part of the target remains, the pre-plasma being generated at a different region from a target region, the different region being located on an incident side of the prepulse laser light; and a main pulse laser light source generating the extreme ultraviolet light by irradiating the pre-plasma with the main pulse laser light.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/569,194, filed on Sep. 29, 2009, now allowed, and claims the benefitof priority from the prior Japanese Patent Applications No. 2008-250744,filed on Sep. 29, 2008, and No. 2009-212884, filed on Sep. 15, 2009; theentire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an extreme ultraviolet light sourceapparatus generating an extreme ultraviolet light by irradiating aliquid metal target with a main pulse laser light after irradiating thetarget with a prepulse laser light, and a method of generating anextreme ultraviolet light.

2. Description of the Related Art

In recent years, along with a progress in miniaturization ofsemiconductor device, miniaturization of transcription pattern used inphotolithography in a semiconductor process has developed rapidly. Inthe next generation, microfabrication to the extent of 70 nm to 45 nm,or even to the extent of 45 nm and beyond will be required. Therefore,in order to comply with the demand of microfabrication to the extent of32 nm and beyond, development of such on exposure apparatus combining anextreme ultraviolet (EUV) light source for a wavelength of about 13 nmand a reflection-type reduction projection optical system is expected.

As the EUV light source, there are three possible types, which are alaser produced plasma (LPP) light source using plasma generated byirradiating a target with a laser beam, a discharge produced plasma(DPP) light source using plasma generated by electrical discharge, and asynchrotron radiation (SR) light source using orbital radiant light.Among these light sources, the LPP light source has such advantages thatluminance can be extremely high as close to the black-body radiationbecause plasma density can be made higher, luminescence only with adesired wavelength band is possible by selecting a target material,there is no construction such as electrode around a light source becausethe light source is a point light source with nearly isotropic angulardistributions, and extremely wide collecting solid angle can beacquired, and so on. Accordingly, the LPP light source is expected as alight source for EUV lithography which requires more than several dozento several hundred watt power.

In the EUV light source apparatus with the LPP system, firstly, a targetmaterial supplied inside a vacuum chamber is irradiated with a laserlight to be ionized and thus generate plasma. Then, a cocktail lightwith various wavelength components including an EUV light is emittedfrom the generated plasma. Consequently, a desired wavelength component,which is a component with a 13.5 nm wavelength, for instance, isreflected and collected using an EUV collector mirror which selectivelyreflects the EUV light with the desired wavelength, and inputted to anexposure apparatus. On a reflective surface of the EUV collector mirror,a multilayer coating with a structure in that thin coating of molybdenum(Mo) and thin coating of silicon (Si) are alternately stacked, forinstance, is formed. The multilayer coating has a high reflectance ratio(of about 60% to 70%) for the EUV light with a 13.5 nm wavelength.

Japanese patent application Laid-Open No. H11-250842 discloses a laserplasma light source which is a light source with a high conversionefficiency due to previously forming a trench on a solid target,gasifying a surface portion of inside the trench by irradiating thetrench with an ablation laser, and thermal ionizing the gasifiedmaterial by irradiating this material with a heating laser light. Here,conversion efficiency means a ratio of power of a generated EUV lightwith a desired wavelength to power of a laser light that entered atarget. This light source is suitable for temporary observation of theextreme ultraviolet light because the conversion efficiency can be madehigher. However, because of using a bulk material, it is difficult tocontinuously form the same trenches over a long time and continuouslysupply the solid targets to the trenches. Therefore, it is difficult touse the light source as a light source for the exposure apparatus thatis required to drive stably over a long time.

Published Japanese Translation No. 2005-525687 of the PCT InternationalPublication discloses a certain apparatus. In this apparatus, a secondtarget is generated by irradiating a first target at a gaseous statewhich could be noble gas such as Xe, the noble gas being gas underordinary temperature that is discharged from a nozzle by a pressure,with a first energy pulse. Then, the second targetisotropically-diffused with time is irradiated with a second energypulse in order to generate a plasma, and a radial ray is outputted fromthe generated plasma. However, in the case where the Xe target is used,because luminance efficiency of the desired EUV light with a 13.5 nmwavelength is low, the conversion efficiency becomes low (under 1%).Therefore, it is difficult to use such apparatus using the Xe target asa light source for the exposure apparatus.

On the other hand, US patent application Laid-Open No. 2008/0149862discloses a laser light source using a liquid droplet C300 of Sn whichis able to efficiently emit a 13.5 nm EUV light as a target. In thislaser light source, firstly, as shown in FIG. 1( a), a target 300 beinga liquid droplet is broken and expanded by a prepulse P300. After that,the expanded target 301 is irradiated with a main pulse P301, and an EUVlight is generated. In this light source, it is possible to improve theconversion efficiency of the EUV light by using the Sn as a liquiddroplet but not solid, and irradiating the shattered and expanded targetwith the main pulse. Thereby, the light source is able to drive stablyover a long time as the laser light source for the exposure apparatus.Moreover, the conversion efficiency is improved because of using aliquid metal but not gaseous as a target.

However, when the droplet being a liquid metal is irradiated with theprepulse, the droplet will be flicked off from a droplet moving axis tobe broken. Therefore, a plasma, gasified droplets and tiny droplets, orthe like, will drift along an optical axis of the prepulse laser whileexpanding, and a lot of debris can be generated. Furthermore, also dueto irradiating these drifted and expanded targets with the main pulseeven more debris can be generated and fly off, if a diameter and a yieldof the tiny droplets is large. The flying debris contaminate or damageneighboring optical elements such as a reflecting surface of an EUVcollector mirror, for instance, and thus can decrease a reflecting ratiowith respect to a 13.5 nm EUV light in a short time. Therefore, there isa problem that a light source that can be reliable for a long time cannot be provided.

In addition, because the flying clustered tiny droplets are neutralparticles, flight thereof can not be controlled by generating anelectromagnetical field, or the like. Moreover, the difficulties ofcontrolling a position and distribution of such flight of targets makesit difficult to adjust a spot diameter of the main pulse to the targets.Therefore, as described above, most tiny droplets are not irradiatedwith the main pulse but fly off around directly as debris.

BRIEF SUMMARY OF THE INVENTION

It is therefore an object of the present invention to resolve theabove-described problems and to provide an extreme ultraviolet laserlight apparatus which is able to stably generate reliable extremeultraviolet lights for a long time while maintaining a high conversionefficiency, and a method of generating an extreme ultraviolet light.

In accordance with one aspect of the present invention, an extremeultraviolet light source apparatus, which is to be generate an extremeultraviolet light by irradiating a target with a main pulse laser lightafter irradiating the target with a prepulse laser light, the extremeultraviolet light source apparatus comprises: a prepulse laser lightsource generating a pre-plasma by irradiating the target with theprepulse laser light while a part of the target remains, the pre-plasmabeing generated at a different region from a target region, thedifferent region being located at an incident side of the prepulse laserlight; and a main pulse laser light source generating the extremeultraviolet light by irradiating the pre-plasma with the main pulselaser light.

In accordance with another aspect of the present invention, a method ofgenerating an extreme ultraviolet light by irradiating a target with amain pulse laser light after irradiating the target with a prepulselaser light, comprises the steps of: a pre-plasma generating step wherea pre-plasma is generated by irradiating the target with the prepulselaser light while a part of the target remains, the pre-plasma beinggenerated at a different region from a target region, the differentregion being located on an incident side of the prepulse laser light;and an extreme ultraviolet light generating step where the extremeultraviolet light is generated by irradiating the pre-plasma with themain pulse laser light.

These and other objects, features, aspects, and advantages of thepresent invention will become apparent to those skilled in the art fromthe following detailed description, which, taken in conjunction with theannexed drawings, discloses preferred embodiments of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a luminescent state of an extremeultraviolet light when a prior art extreme ultraviolet light sourceapparatus is used;

FIG. 2 is a schematic view showing a concept of a luminescent process ofan extreme ultraviolet light by an extreme ultraviolet light sourceapparatus according to a first embodiment of the present invention;

FIG. 3 is a schematic view showing a relationship between debris flyingfrom a plasma after EUV luminescence and a residual droplet;

FIG. 4 is a schematic view showing a structure of the extremeultraviolet light source apparatus according to the first embodiment ofthe present invention;

FIG. 5 is a schematic view showing a structure of an extreme ultravioletlight source apparatus according to a second embodiment of the presentinvention;

FIG. 6 is a schematic view showing a position control system forprepulse (phase 1);

FIG. 7 is a schematic view showing the position control system forprepulse (phase 2);

FIG. 8 is a schematic view showing the position control system forprepulse (phase 3);

FIG. 9 is a schematic view showing a timing control system for droplet,prepulse and main pulse;

FIG. 10 is a view of operation statuses of an EUV light luminescence ina case where a wire target is used as a solid target for an extremeultraviolet light source apparatus according to a third embodiment ofthe present invention;

FIG. 11 is a view of operation statuses of an EUV light luminescence ina case where a disk target is used as a solid target for an extremeultraviolet light source apparatus according to an alternate example 1of the third embodiment of the present invention;

FIG. 12 is a view of operation statuses of an EUV light luminescence ina case where a disk target is used as a solid target for an extremeultraviolet light source apparatus according to an alternate example 2of the third embodiment of the present invention;

FIG. 13 is a view of operation statuses of an EUV light luminescence ina case where a tape target is used as a solid target for an extremeultraviolet light source apparatus according to an alternate example 3of the third embodiment of the present invention;

FIG. 14 is a view of operation statuses of an EUV light luminescence ina case where a cylindrical target is used as a solid target for anextreme ultraviolet light source apparatus according to an alternateexample 4 of the third embodiment of the present invention;

FIG. 15 is a view of operation statuses of an EUV light luminescence ina case where a trench target is used as a solid target for an extremeultraviolet light source apparatus according to an alternate example 5of the third embodiment of the present invention;

FIG. 16 is a schematic view showing a structure of a prepulse laser foran extreme ultraviolet light source apparatus according to a fourthembodiment of the present invention; and

FIG. 17 is a schematic view showing a structure of a prepulse laser foran extreme ultraviolet light source apparatus according to an alternateexample 1 of the fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments of an extreme ultraviolet light source apparatusaccording to the present invention will be described below in detailwith reference to the accompanying drawings.

Firstly, a concept in an aspect of the present invention will bedescribed in detail with reference to FIGS. 2 and 3. In FIG. 2, adroplet D of Sn being a liquid metal is discharged from a nozzle along adroplet axis C1 as a target. As shown in FIG. 2( a), the droplet D1arrived at a predetermined position is irradiated with a prepulse P1emitted from a prepulse laser, and as shown in FIG. 2( b), the dropletD1 becomes a pre-plasma 1 due to ablation by the prepulse P1. Here, thepre-plasma 1 is assumed as being in a weak plasma state to such extentthat an part near a irradiated surface of the droplet does not emit anEUV light or being in a mixed state of a neutral gas (steam) and theweak plasma. In the following descriptions and drawings, the weak plasmastate and the mixed state of the neutral gas and the weak plasma will becalled as the pre-plasma.

A non-broken residual droplet D2 which is a non-ablated part of thedroplet remains while the pre-plasma 1 is generated. Here, thepre-plasma 1 is generated in a direction opposite to an irradiationdirection of the prepulse P1, whereas the residual droplet D2 does notmove so much from the droplet axis C1. Therefore, a region of theresidual droplet D2 and a region of the pre-plasma 1 will be at a stateseparated from each other. After a short time Δt from irradiation of theprepulse P1, as shown in FIG. 2( b), a desired 13.5 nm EUV light 2 willbe isotropically emitted from the main pulse laser, as a main pulse P2is irradiated only to the pre-plasma 1. After that, as shown in FIGS. 2(c) and 3, the residual droplet D2 that remains after the generation ofthe desired EUV light moves to a droplet moving direction DAapproximately along the droplet axis C1.

Here, fragment that can be caused by the droplet being broken by theprepulse P1 is not caused in the residual droplet D2, and generation ofdebris will not occur because the residual droplet 2 is not irradiatedwith the main pulse P2. Furthermore, the region of the pre-plasma 1 isirradiated with the main pulse P2, and the EUV light 2 is emitted fromthe irradiated region. As a result, because debris 3 is generated onlyfrom the region of the pre-plasma 1, it is possible to enormously reducea yield of the debris 3, and it is possible to enormously reducecontaminations on reflective surfaces of neighboring optical elementssuch as the EUV collector mirror, and so on, for instance. By thisarrangement, a light source being reliable for a long time can beprovided.

In addition, the inventors of the present invention find out that it ispossible to obtain a conversion efficiency which is approximately equalto the conversion efficiency according to the light source apparatusshown in FIG. 1 even if only the pre-plasma 1 is irradiated with themain pulse P2. In this case, a laser intensity of the prepulse P1 isequal to or less than 1×10⁹ W/cm² but does not fall bellow 1×10⁷ W/cm²in order to obtain a desired EUV light 2. In addition, a YAG laser witha wavelength of 1.064 μm is used for the prepulse laser, and a CO₂ laserwith a wavelength of 10.6 μm is used for the main pulse laser. Arepetition rate is equal to or more than 7 kHz. Accordingly, the dropletD is also discharged synchronously at one or more times the repetitionrate.

Here, a spot diameter of the prepulse P1 with respect to the droplet D1is equal to or less than a diameter of the droplet D1, and the spotdiameter of the main pulse P2 with respect to the pre-plasma 1 isapproximately equal to the diameter of the pre-plasma 1. Moreover, aprepulse irradiating direction P1A of the prepulse P1 is a differentdirection from a main-pulse irradiating direction P2A of the main pulseP2, and focusing positions of the prepulse P1 and the main pulse P2differ from each other. Unlike the case of the prior art (FIG. 1), thefocusing position of the main pulse P2 is located near a center of thepre-plasma 1 generated at a front of the incident side of the prepulselaser on the droplet D1.

Because the pre-plasma 1 includes ions, the pre-plasma 1 can becontrolled to have a position and distribution of the space of thepre-plasma 1 controlled by using an electromagnetical field.

First Embodiment

Next, an extreme ultraviolet light source apparatus according to a firstembodiment of the present invention will be described in detail withreference to FIG. 4. In FIG. 4, the extreme ultraviolet light sourceapparatus has a nozzle 10 to discharge droplets D of Sn into a vacuumchamber (not shown), a prepulse laser 11 being a YAG laser which emitsthe prepulse P1 with a 1.064 μm wavelength to the droplet D1 inside thevacuum chamber at the repetition rate of 7 kHz or over, and a main pulselaser 21 being a CO₂ laser which emits the main pulse laser P2 with a10.6 μm wavelength to the pre-plasma 1 generated inside the vacuumchamber at the repetition rate of 7 kHz or over which is the same withthe prepulse P1. Intensity of the prepulse P1 is equal to or greaterthan 1×10⁷ W/cm² but not exceeding 1×10⁹ W/cm². The intensity of theprepulse P1 is to the extent that enables generation of the pre-plasma 1which is capable of emitting the desired and sufficient 13.5 nm EUVlight 2 from a part of the droplet D, and generation of the residualdroplet D2 which is a part of the non-broken droplet D.

The prepulse laser 11 generates the pre-plasma 1 outside a region of theresidual droplet D2 by irradiating the droplet D1 arriving at apredetermined position with the prepulse P1 via a mirror 12 and afocusing mirror 13.

On the other hand, the main pulse laser 21 generates the EUV light 2 byirradiating only the pre-plasma 1 generated by the prepulse P1 via amirror 22, an off-axis parabolic focusing mirror 23, an aperture 24 a ofan EUV collector mirror 24 of which reflective surface is an ellipticalsurface. The main pulse P2 is finally absorbed by a dumper 25.

The EUV collector mirror 24 with the elliptical surface collects thegenerated EUV light 2, and the collected EUV light 2 is inputted to aside of the exposure apparatus outside the chamber (not shown).

A trigger generator 31 generates generating timing of the prepulse P1 inthe prepulse laser 11, generating timing of the main pulse P2 in themain pulse laser 21, and discharge timing of the droplet D from thenozzle 10 by adjustment under control of a main controller 30. In thisarrangement, the trigger generator 31 synchronizes irradiating timing ofthe prepulse P1 and the main pulse P2 at a predetermined space positionof the droplet D1 while conforms the discharge rate of the droplet D tothe repetition rate of the prepulse P1 and the main pulse P2.

An image analyzer 34 takes an image of a reflected light from thedroplet D1 illuminated by a backlight 35 and an image of an illuminationof the pre-plasma 1 using a CCD camera 33, and analyzes the opticalreceived image. A focusing position controller 32 controls focusingpositions and directions of the focusing mirrors 13 and 23 based on theanalysis result from the image analyzer 34.

In addition, the residual droplet D2 is retrieved by a debris catcher 40mounted in the droplet moving direction DA.

In such the structure, the pre-plasma 1 is generated at a front of theincident side of the prepulse P1 on the droplet D1 by irradiating thefalling droplet D1 with prepulse P1, and after the shot time Δt, the EUVlight 2 is generated by irradiating the pre-plasma 1 with the main pulseP2. The generated EUV light 2 is outputted outside via the EUV collectormirror 24. In addition, an optimum range of the short time Δt is 50 nsto 100 ns. As described above, because only minimal debris from thepre-plasma 1 are generated due to the residual droplet D2 not beingirradiated with the main pulse p2, it is possible to enormously reducecontaminations on the reflective surface of the EUV collector mirror 24,etc. due to the debris, and it is possible to provide a light sourcebeing reliable for a long time. Furthermore, because the droplet of theSn being a liquid metal is used and the pre-plasma 1 is effectivelyirradiated with the main pulse P2, it is possible to attain a highconversion efficiency which is the same with the case where the expandedtarget being generated by crashing the whole droplet using the prepulseP1 is irradiated with the main pulse.

Second Embodiment

In the first embodiment described above, the case where the prepulse P1is emitted in the direction approximately perpendicular to the dropletaxis C1 has been described. On the other hand, in the second embodiment,as shown in FIG. 5, the prepulse P1 is emitted in a directionapproximately along the droplet axis C1.

By this arrangement, due to the debris 3 from the pre-plasma 1 beinggenerated along the droplet axis C1 after the pre-plasma 1 is irradiatedwith the main pulse P2, the debris catcher 40 can easily retrieve notonly the residual droplet D2 but also the debris 3 generated from thepre-plasma 1. In this case, an aperture of the debris catcher 40 can bedesigned based on how the debris 3 expand from the droplet axis C1.Accordingly, it is possible to keep an amount of the flying debris 3 toa minimum. In addition, as shown in FIG. 5, due to the EUV collectormirror 24 being mounted in the direction perpendicular to a flyingdirection of the debris 3, i.e. the moving direction of the droplet D,it is possible, particularly, to effectively prevent the reflectivesurface of the EUV collector mirror 24 from contamination by debris.

Position Control Example of Prepulse

Here, a position control of the prepulse P1 outputted from the prepulselaser 11 will be described. As shown in FIG. 6, a luminous point whichis specified from the image of the luminescence of the pre-plasma 1taken by the CCD camera 33 may be determined as a focus position of theprepulse P1. In this case, the position control of the prepulse P1 isexecuted as the image analyzer 34 specifies the luminous point of thepre-plasma 1 and the focusing point controller 32 controls a triaxialstage of the focusing mirror 13 such that the specified position becomesa desired focusing position of the prepulse P1.

Moreover, as shown in FIG. 7, the position control of the prepulse P1may be executed using a laser interferometer. In this embodiment, aMach-Zehnder interferometer is used. In particular, a partial reflectionmirror M1, a total reflection mirror M2, a total reflection mirror M3,and a partial reflection mirror M4 are mounted between a laser 50 as alight source and a lens 33 a in order to form two optical paths, oneoptical path passing through the laser 50, a partial reflection mirrorM1, the total reflection mirror M2 and the partial reflection mirror M4,and the other optical path passing through the laser 50, the partialreflection mirror M2, the total reflection mirror M3 and the partialreflection mirror M4. In this arrangement, interference between the twooptical paths will be measured. Subsequently, the laser interferometeris arranged so that the pre-plasma 1 is located on the one optical path,i.e. the pass between the total reflection mirror M2 and the partialreflection mirror M4.

The CCD camera 33 takes an image of an interference pattern occurred byexistence of the pre-plasma 1 via the lens 33 a. The image analyzer 34analyzes the imaged interference pattern. The focusing positioncontroller 32 specifies a position for making a density of thepre-plasma 1 become a density to which the EUV luminous efficiency bythe main pulse P2 is optimal as the focus position of the prepulse P1based on the interference pattern that depends on a density of theregion of the pre-plasma 1, and controls the triaxial stage of thefocusing mirror 13 so that the specified position becomes a desiredfocus position of the prepulse P1. By such processes, the positioncontrol of the focus position of the prepulse P1 is executed. Here, whena main laser is a CO₂ laser, a high conversion efficiency can beobtained if the density of the pre-plasma is approximately 10¹⁸/cc.

In addition, as shown in FIG. 7, it is preferable that a light source ofthe interferometer is a laser. The image taken by the CCD camera 33 isanalyzed as the interference pattern caused by the spatial distributionof refractive index that depends on the density of the pre-plasma 1, asdescribed above. This is because, by such arrangement that a fringenumber with respect to change of refractive index becomes dense byshortening a wavelength of the light source of the interferometer, it ispossible to measure a plasma with a weaker density state.

Furthermore, as shown in FIG. 8, the focusing position of the prepulseP1 may be controlled based on observing scattering light using the CCDcamera 33, the scattering light occurring by the pre-plasma 1 beingirradiated with a probe laser 51. In this case, the shorter thewavelength and the higher the intensity of the probe laser 51 are, thehigher the intensity of the scattering light from the pre-plasma 1becomes. Therefore, a signal intensity detected by the CCD camera 33becomes higher, whereby it is possible to take an image of which S/Nratio is larger. Herewith, the position control of the focus position ofthe prepulse P1 is executed by a way that the image analyzer 34 analyzesthe position and the distribution of the pre-plasma 1 based on the imagetaken by the CCD camera 33, and the focusing position controller 32specifies the focus position of the prepulse P1 based on the analysisresult and controls the triaxial state of the focusing mirror 13 suchthat the specified focus position becomes the desired focus position.

However, if the prepulse P1 is outputted at a repetition rate equal toor greater than over 7 to 100 kHz, the position control with analyzingall of the repeated images is not realistic. Actually, it is possible toexecute the position control while feed backing an average ranging fromseveral micro seconds to several seconds to a triaxial stage 14.

A single CCD camera 33 is used for the position control of the prepulseP1 in FIGS. 6 to 8. However, because the focus position is athree-dimensional location, at least two CCD cameras are required.

In the embodiment shown in FIGS. 6 to 8, the off-axis parabolic focusingmirror is used as an optical focusing system of the prepulse laser.However, the present invention is not limited to this example. The focusposition of the prepulse may be controlled by using a focusing lens,locating a highly-reflective mirror ahead or behind the focusing lens,and locating the highly-reflective mirror on the triaxial stage.

Timing Control Example

Next, timing control of the droplet D, the prepulse P1 and the mainpulse P2 will be described in detail. FIG. 9 is a block diagram showinga structure of a timing control system. In FIG. 9, a piezoelectricelement 61 for discharging a liquid metal as a droplet is mounted at thenozzle 10. This piezoelectric element 61 is connected to the maincontroller 30 and controls discharge timing of the droplet D. Thepiezoelectric element 61 discharges the droplet D at a predeterminedrate by making an end of the nozzle 10 vibrate in the droplet movingdirection DA. The rest of the structure is the same as the structureshown in FIG. 4, and the same reference numbers will be used forreferring to the portions that are the same as the portions shown inFIG. 4. In addition, the CCD camera 33 and the image analyzer 34 areconstructing an observation system for the prepulse P1, and the CCDcamera 53 and the image analyzer 54 are constructing an observationsystem for the main pulse P2.

Firstly, at timing of T=0, a control signal for discharging the dropletD is transmitted from a piezoelectric controller 60 to the piezoelectricelement 61, and the droplet D is discharged from a position f0. Thedroplet D arrives at a focus position f1 after a predetermined time ΔT1from the timing T=0. Here, a trigger generator 31 a may be controlled sothat the droplet D is irradiated with the prepulse P1 at that moment.

However, every droplet D does not necessarily arrive the focus positionf1 of the prepulse P1 after the same time period from discharging. Thisis because of instability of the droplet D, and occurrence of long-timetransformation of the droplet D caused by thermal transformation ofstructures such as the chamber (not shown), and so on. That is, due tovarious causes, there are possibilities of traveling times of thedroplets D from the discharge position f0 to the focus position f1 ofthe prepulse P1 changing. Therefore, it is necessary to monitor thearrival of the droplet D at the focus position P1 of the prepulse f1,and control the irradiating timing of the prepulse P1 in order toirradiate the droplet D with the prepulse P1 at the focus position f1 ofthe prepulse P1.

For this reason, the image of the plasma luminescence at a time of thedroplet D being irradiated with the prepulse P1 is taken using the CCDcamera 33, and an oscillating timing of the prepulse P1 is changed viathe trigger generator 31 a such that the plasma luminescence can beconstantly detected as being approximately the same intensity. The imageanalyzer 34 analyses the intensity of the plasma luminescence based onthe image transmitted from the CCD camera 33, compares the analyzedintensity and an intensity of a plasma luminescence detected atappropriate timing, which is preliminarily stored, and determines as towhether the irradiating timing of the prepulse P1 is appropriate or not.The main controller 30 changes the timing of the trigger generator 31 abased on the result of the determination about the irradiating timing,and controls to change the irradiating timing of the prepulse P1 tocontrol the irradiating timing to become adequate.

Likewise, an image of the luminescence of the pre-plasma 1 generatedfrom the droplet D at timing T (i.e. after a time ΔT2 from the dischargetiming of the droplet D) is taken by the CCD camera 33, and theirradiating timing of the main pulse P2 is controlled by the maincontroller 30 via the trigger generator 31 b based on the result of thedetermination about the intensity of the plasma luminescence by theimage analyzer 54, so that the pre-plasma 1 is irradiated with the mainpulse P2 at the focus position f2. Here, in the case of the main pulseP2, the CCD camera 53 may directly detect the intensity of the EUVlight.

In addition, in the embodiment described above, the droplet of theliquid metal Sn has been explained as an example. However, the presentinvention is not limited to such arrangement. Any kind of liquid metalmay be used for the droplets. Moreover, although the case where theirradiating prepulse laser 11 is a 1.064 μm YAG laser and the main pulselaser 21 is a 10.6 μm CO₂ laser has been described, any irradiatinglaser can be used as long as the laser is able to make a droplet producea desired EUV light.

Third Embodiment

In the first and second embodiments described above, the droplet asbeing a liquid target is used as a target. On the other hand, in a thirdembodiment, a solid target is used as a target.

FIG. 10 is a view of operation statuses of an EUV light luminescence ina case where a wire target is used as a solid target. The wire target100 has a structure in that a core wire 101 being a piano wire (a steelwire made of a high-carbon steel) of high strength is coated with atarget material 102 of Sn. The target material 102 can be formed byhaving the core wire 101 coated with a Sn metal by hot-dip plating whilehaving the core wire 101 dunk in the fused Sn metal.

As with the droplet D moving along the droplet axis C1, the wire target100 moves along a target axis C10 in a single direction, and isirradiated with the prepulse P1 at a predetermined cycle (shown in FIG.10( a)). After that, a part of the target material 102 irradiated withthe prepulse P1 generates the pre-plasma 1, a strong plasma is generatedby irradiating the pre-plasma 1 with the main pulse P2 at thepredetermined cycle, and the EUV light 2 is emitted from the strongplasma (shown in FIG. 10( b)).

The wire target 100 is let out from a drum by which the wire target 100is being preliminary taken up, and the used wire target 100 which has aresidual target material 102 remaining on the core wire 101 is taken upby another drum. The used wire target 100 can be put off together withthis another drum.

In addition, it is possible to reuse the wire target 100 by looping thewire target 100. In this case, the used wire target 100 is recycled intoa new wire target 100 by cooling down the heated target material 102 onthe used wire target 100 using a cooling function, and recoating orsupplementarily coating the core wire 101 with a target material 102using a recycling system.

In the case of using the wire target 100, due to the debris changingfrom solid to liquid and finally to gas, thermal energy (latent heat ina shift from solid to liquid phase) is required unlike the case of thedroplet D, and thus, a yield of neutral particles can be reduced.Moreover, optimization of a density state of the pre-plasma 1 generatedfrom the wire target 100 can be controlled more easily compared with thecase of generating the pre-plasma 1 from the droplet D, and it makes itpossible to increase the conversion efficiency (CE).

Alternate Example 1 of Third Embodiment

In this alternate example 1, a disk target 110 is used as the target inplace of the wire target 100. As shown in FIG. 11, in the disk target110, a rim of the disk 111 is coated with a target material 112 of Sn.When the target material 112 is irradiated with the prepulse P1 from anexternal diameter direction at the predetermined cycle in a state thatthe disk target 110 is rotating around an axis C20 as being a rotatingcenter (shown in FIG. 11( a)), the pre-plasma 1 is generated. Byirradiating this pre-plasma 1 with the main pulse P2 at thepredetermined cycle, strong plasma is generated and the EUV light 2 isemitted (shown in FIG. 11( b)).

In this case, by arranging that a trajectory of the prepulse P1 on therim of the disk target 110 to be irradiated with the prepulse P1 becomesa volute, it becomes possible to provide a comparatively long timeluminescence of the EUV light 2. As is obvious, by recycling the targetmaterial 112 using the cooling system and the recycling system, it ispossible to provide a continuous luminescence of the EUV light 2.

In this alternate example 1, in the case of using the disk target 110,due to the debris changing from solid to liquid and finally to gas,thermal energy (latent heat in a shift from solid to liquid phase) isrequired unlike the case of the droplet D, and thus, a yield of neutralparticles can be reduced. Moreover, optimization of a density state ofthe pre-plasma 1 generated from the disk target 110 can be controlledmore easily compared with the case of generating the pre-plasma 1 fromthe wire target 100, and it makes it possible to increase the conversionefficiency (CE).

Alternate Example 2 of Third Embodiment

In this alternate example 2, although a disk target 120 with a diskshape like in the case of the alternate example 1 is used, a disk 121 iscoated with a target material 122 of Sn not on the rim but on a mostouter circumference of one side thereof. When the target material 122 isirradiated with the prepulse P1 from around an axis C30 at thepredetermined cycle in a state that the disk target 120 is rotatingaround the axis C30 as being as a rotating center (shown in FIG. 12(a)), the pre-plasma 1 is generated. By irradiating this pre-plasma 1with the main pulse P2 at the predetermined cycle, strong plasma isgenerated and the EUV light 2 is emitted (shown in FIG. 12( b)).

In this case, by arranging that a trajectory of the prepulse P1 on thetarget material 122 to be irradiated with the prepulse P1 becomesspiral, it becomes possible to provide a comparatively long timeluminescence of the EUV light 2. As is obvious, by recycling the targetmaterial 122 using the cooling system and the recycling system, it ispossible to provide a continuous luminescence of the EUV light 2.

In this alternate example 2, in the case of using the disk target 120,due to the debris changing from solid to liquid and finally to gas,thermal energy (latent heat in a shift from solid to liquid phase) isrequired unlike the case of the droplet D, and thus, a yield of neutralparticles can be reduced. Moreover, optimization of a density state ofthe pre-plasma 1 generated from the disk target 120 can be controlledmore easily compared with the case of generating the pre-plasma 1 fromthe wire target 100, and it makes it possible to increase the conversionefficiency (CE).

Alternate Example 3 of Third Embodiment

In this alternate example 3, a tape target 130 is used as a solidtarget. As shown in FIG. 13, in the tape target 130, one wide face of acore portion 131 is coated with a target material 132 of Sn. When thetarget material 132 is irradiated with the prepulse P1 from anapproximate perpendicular direction to a face of the target material 132at the predetermined cycle in a state that the disk target 130 isrotating around the axis C30 as being as a rotating center (shown inFIG. 13( a)), the pre-plasma 1 is generated. By irradiating thispre-plasma 1 with the main pulse P2 at the predetermined cycle, strongplasma is generated and the EUV light 2 is emitted (shown in FIG. 13(b)).

In this case, when the tape target 130 is nonreusable, by moving thetape target 130 in a loop so that a trajectory of the prepulse P1 on thetarget material 132 to be irradiated with the prepulse P1 issequentially shifted in a direction perpendicular to a target axis C40,it is possible to provide a comparatively long time luminescence of theEUV light 2. As is obvious, by recycling the target material 132 usingthe cooling system and the recycling system while looping the tapetarget 130, it is possible to provide a continuous luminescence of theEUV light 2.

In this alternate example 3, in the case of using the tape target 130,due to the debris changing from solid to liquid and finally to gas,thermal energy (latent heat in a shift from solid to liquid phase) isrequired unlike the case of using the droplet D, and thus, a yield ofneutral particles can be reduced. Moreover, optimization of a densitystate of the pre-plasma 1 generated from the tape target 130 can becontrolled more easily compared with the case of generating thepre-plasma 1 from the wire target 100, and it makes it possible toincrease the conversion efficiency (CE).

Alternate Example 4 of Third Embodiment

In this alternate example 4, a cylindrical target 140 is used as a solidtarget. As shown in FIG. 14, a circumference of a cylindrical coreportion 141 is coated with a target material 142 of Sn. When the targetmaterial 142 is irradiated with the prepulse P1 from an approximateperpendicular direction to a face of the target material 142 at thepredetermined cycle in a state that the cylindrical target 140 isrotating around the axis C50 as being a rotating center (shown in FIG.14( a)), the pre-plasma 1 is generated. By irradiating this pre-plasma 1with the main pulse P2 as the predetermined cycle, strong plasma isgenerated and the EUV light 2 is emitted (shown in FIG. 14( b)).

In this case, by arranging that a trajectory of the prepulse P1 on thetarget material 142 being to be irradiated with the prepulse P1 becomesvolute, it is possible to provide a comparatively long time luminescenceof the EUV light 2. As is obvious, by recycling the target material 142using the cooling system and the recycling system, it is possible toprovide a continuous luminescence of the EUV light 2.

In this alternate example 4, in the case using the cylindrical target140, due to the debris changing from solid to liquid and finally to gas,thermal energy (latent heat in a shift from solid to liquid phase) isrequired unlike the case of the droplet D, a yield of neutral particlescan be reduced. Moreover, optimization of a density state of thepre-plasma 1 generated from the cylindrical target 140 can be controlledmore easily compared with the case of generating the pre-plasma 1 fromthe wire target 100, and it makes it possible to increase the conversionefficiency (CE).

Alternate Example 5 of Third Embodiment

In this alternate example 5, a dimple target 150 having dimples is usedas a solid target. As shown in FIG. 15, the dimple target 150 is thesame as the tape target 130 shown in FIG. 13, expect that the dimples153 are formed on a front surface of the dimple target 150. That is, inthe dimple target 150, the dimples 153 formed on one wide front face ofa tape-type core portion 151 are coated with the target materials 152.The dimples 153 are arrayed along a direction of a target axis C60.Although the dimples 153 are formed on the tape-type target material152, it is possible to form the dimples 153 by having dimples providedon a surface of the core portion 151 while the target material 152 is tobe formed and having the core portion 151 including the dimples coatedwith the target material 152.

When the target material 153 is irradiated with the prepulse P1 from anapproximately perpendicular direction to a face of the target material152 at the predetermined cycle being synchronized with the positions ofthe dimples 153 in a state that the dimple target 150 is moving in thedirection of the target axis C60 (shown in FIG. 15( a)), the pre-plasma1 is generated. By irradiating this pre-plasma 1 with the main pulse P2at the predetermined cycle, strong plasma is generated and the EUV light2 is emitted (shown in FIG. 15( b)).

In this alternate example 5, in the case of using the dimple target 150,due to the debris changing from solid to liquid and finally to gas,thermal energy (latent heat in a shift from solid to liquid phase) isrequired unlike the case of the droplet D, a yield of neutral particlescan be reduced. Moreover, optimization of a density state of thepre-plasma 1 generated from the dimple target 150 can be controlled moreeasily compared with the case of generating the pre-plasma 1 from thetape target 130, and it makes it possible to increase the conversionefficiency (CE).

In addition, the dimples 153 as described in the alternate example 5 canbe applied to anyone of the above described third embodiment andalternate examples 1 to 4 of the third embodiment. Forming these dimples153 enables generation of a higher density pre-plasma 1, whereby theconversion efficiency (DE) can be further increased.

Furthermore, in each of the alternate examples 1 to 5, the whole targetcan be made from Sn material. For instance, in the alternate example 1,the whole disk target 110 can be made from Sn.

Fourth Embodiment

Next, a fourth embodiment of the present invention will be described indetail. In the fourth embodiment, a picosecond pulse laser that outputsa pulse of which pulse time width becomes equal to or less than apicosecond is used as the prepulse laser 11 shown in FIG. 4. Here, thepicosecond pulse laser outputting a pulse of which pulse time width T isequal to or less than a picosecond means a pulse laser of which pulsewidth T is less than 1 ns (T<1 ns) .

FIG. 16 is a schematic diagram showing a structure of the prepulse laseraccording to the fourth embodiment of the present invention. In FIG. 16,the prepulse laser 11A is a self-mode-locking titanium-sapphire laser.The prepulse laser 11A has a titanium-sapphire crystal 203 as being alaser agency arranged in between face-to-face high reflective mirrors201 and 202. An excitation light LA oscillated at an excitation lightsource 200 enters the titanium-sapphire crystal 203 via the highreflective mirror 201. The excitation light source 200 outputs a secondharmonic (532 nm) of a semiconductor laser excitation Nd:YVO4 or asemiconductor excitation Nd:YAG as the excitation light LA. Thetitanium-sapphire crystal 203 is optically excited by the excitationlight LA, and emits a light with 800 nm, for instance. The highreflective mirror 201 lets the excitation light LA pass through andhighly reflects the light emitted from the titanium-sapphire crystal203. On the other hand, the reflective mirror 202 highly reflects thelight emitted from the titanium-sapphire crystal 203.

A semiconductor supersaturation absorbing mirror 205 and an outputcoupling mirror 208 form a laser resonator. The light reflected by thehigh reflective mirror 201 is collected on the semiconductorsupersaturation absorbing mirror 205 by a concave high reflective mirror204. On the other hand, the light reflected by the high reflectivemirror 202 is waveguided to the output coupling mirror 208 throughprisms 206 and 207. The output coupling mirror 208 outputs a part oflight inside the laser resonator to the outside as the prepulse P1.

When the titanium-sapphire crystal 203 is irradiated with the excitationlight LA from the excitation light source 200 via the high reflectivemirror 201, a longitudinal-mode in the laser resonator oscillates whilemode-locking due to supersaturation absorption of the semiconductorsupersaturation absorbing mirror 205, and an inverted distributionenergy accumulated in the titanium-sapphire crystal 203 is outputtedfrom the output coupling mirror 208 as an optical pulse (the prepulseP1) having been concentrated within a short time (picosecond).

When the target such as the droplet D is irradiated with the prepulse P1of which time width is equal to or less than a picosecond, only a thinsurface of the target is ionized to generate the pre-plasma because thetime width is an enormously short period of time as equal to or lessthan a picosecond. Therefore, in the case of using the droplet D, due tothe target becoming harder to break up, dispersion and debris of thetarget can be reduced. Moreover, also in the case where the target isthe solid target, dispersion and debris of the target can be reducedwhile breakage inside the target can be prevented at the same time.Furthermore, because such pulse of which time width is equal to or lessthan a picosecond is generated, a peak power of the pulse becomeshigher. Accordingly, it is possible to generate pre-plasma even withusing a low energy pulse, whereby miniaturization of the device can beenhanced.

Alternate Example 1 of Fourth Embodiment

Here, it is possible to generate the prepulse P1 of which time width isequal to or less than a picosecond even with a fiber laser. FIG. 17 is aschematic diagram showing a structure of a prepulse laser according toan alternate example 1 of the fourth embodiment of the presentinvention. The prepulse laser 11B has a semiconductor supersaturationabsorbing mirror 215 which is mounted on one end of the Yb doped fiber212 being an amplifiable agency via an optical system, and a highreflective mirror 214 mounted on the other end of the Yb doped fiber 212via a grating 213. A laser resonator is formed between the semiconductorsupersaturation absorbing mirror 215 and the high reflective mirror 214.A 915 nm excitation light outputted from a 915 nm excitation lightsource 210 enters the Yb doped fiber 212 via a WDM coupler 211. The Ybdoped fiber 212 is excited by the inputted 915 nm light, and outputs a980 nm light. The 980 nm light is a mode-locked picosecond pulsegenerated by the longitudinal-mode inside the laser resonator locked bythe semiconductor supersaturation absorbing mirror 215 while waveselection is executed via the grating 213, and is outputted via acoupler 216 and an optical isolator 217 as the prepulse P1.

In this alternate example 1, while the same effect as the fourthembodiment benefitted by the picosecond pulse can be achieved, it ispossible to irradiate the target with the prepulse P1 easily and withhigh accuracy because the prepulse P1 can be waveguided using theoptical fiber. Furthermore, because a value of M² which shows a qualityof a lateral-mode of a fiber laser is about 1.2, light focusingperformance can become high, and it is possible to irradiate the targetwith the prepulse P1 with high accuracy even if the target is small.

In addition, if energy of the prepulse P1 equal to or less than apicosecond is small, it is possible to amplify the prepulse P1 using aregenerative amplifier. Moreover, a femtosecond laser outputting a pulseof which pulse time width becomes a femtosecond can be applied toachieve the same effect as in the case of using the picosecond laser.

Moreover, the shorter the wavelength of the prepulse P1 is, the higherthe absorbency index of Sn being the target becomes. Therefore, it isdesirable that the wavelength of the prepulse P1 is as short aspossible. For instance, in a case of using Nd:YAG laser, absorbencyindexes become higher in order of a wavelength of the Nd:YAG laser=1064nm, a wavelength of twice the harmonic thereof=532 nm, a wavelength ofthrice the harmonic thereof=355 nm, and a wavelength of quadruply theharmonic thereof=266 nm, and by adopting the prepulse P1 with a shortwavelength, it becomes possible to generate pre-plasma with highdensity, whereby the conversion efficiency (CE) can be increased.

As explained above, according to each of the above describedembodiments, the prepulse laser light source generates pre-plasma at thedifferent region different from the target region, which is on theincident side of the prepulse laser light, while a part of the targetremains as the target is irradiated with the prepulse laser light, andthe main pulse laser light source generates the extreme ultravioletlight by irradiating the pre-plasma with the main pulse laser light.Thereby, the debris are generated only from the pre-plasma. By thisarrangement, a light source being reliable for a long time and beingable to reduce an yield of debris can be provided.

In addition, the above-mentioned embodiments and the alternate examplescan be arbitrarily combined with one another.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept of the invention as defined by the appended claims and theirequivalents.

1. An extreme ultraviolet light source apparatus, which is to generatean extreme ultraviolet light by irradiating a target with a main pulselaser light after irradiating the target with a prepulse laser light,the extreme ultraviolet light source apparatus comprising: a prepulselaser light source generating a pre-plasma by irradiating the targetwith the prepulse laser light while a part of the target remains, thepre-plasma being generated at a different region from a target region,the different region being located on an incident side of the prepulselaser light; and a main pulse laser light source generating the extremeultraviolet light by irradiating the pre-plasma with the main pulselaser light.
 2. The extreme ultraviolet light source apparatus accordingto claim 1, wherein an irradiation intensity of the prepulse laser lightis equal to or greater than 10⁷ W/cm² but not exceeding 10⁹ W/cm². 3.The extreme ultraviolet light source apparatus according to claim 1,wherein the main pulse laser light source emits the main pulse laserlight only to the pre-plasma.
 4. The extreme ultraviolet light sourceapparatus according to claim 1, wherein the prepulse laser light sourceis a picosecond pulse laser generating a pulse of which time width isequal to or less than a picosecond.
 5. The extreme ultraviolet lightsource apparatus according to claim 1, wherein the target is a droplet,and the prepulse laser light source emits the prepulse laser light withsuch intensity that lets a part of the droplet remain after the dropletis irradiated with the prepulse laser light.
 6. The extreme ultravioletlight source apparatus according to claim 5, further comprising: acatcher with an aperture for retrieving a residual target after thedroplet is irradiated with the prepulse laser light and a remainingdebris in the pre-plasma, the catcher being mounted in a movingdirection of the target, wherein the prepulse laser light sourceirradiates the target with the prepulse from an opposite direction tothe moving direction of the target, the opposite direction approximatelyconforming to a moving axis of the target.
 7. The extreme ultravioletlight source apparatus according to claim 5, wherein an irradiationintensity of the prepulse laser light is equal to or greater than 10⁷W/cm² but not exceeding 10⁹ W/cm².
 8. The extreme ultraviolet lightsource apparatus according to claim 5, wherein the main pulse laserlight source irradiates only the pre-plasma with the main pulse laserlight.
 9. The extreme ultraviolet light source apparatus according toclaim 5, wherein the prepulse laser light source is a picosecond pulselaser generating a pulse of which time width is equal to or less than apicosecond.
 10. The extreme ultraviolet light source apparatus accordingto claim 1, wherein the target is a solid target.
 11. The extremeultraviolet light source apparatus according to claim 10, furthercomprising: a catcher with an aperture for retrieving a residual targetafter the droplet is irradiated with the prepulse laser light and aremaining debris in the pre-plasma, the catcher being mounted in amoving direction of the target, wherein the prepulse laser light sourceirradiates the target with the prepulse from an opposite direction tothe moving direction of the target, the opposite direction approximatelyconforming to a moving axis of the target.
 12. The extreme ultravioletlight source apparatus according to claim 10, wherein an irradiationintensity of the prepulse laser light is equal to or greater than 10⁷W/cm² but not exceeding 10⁹ W/cm².
 13. The extreme ultraviolet lightsource apparatus according to claim 10, wherein the main pulse laserlight source irradiates only the pre-plasma with the main pulse laserlight.
 14. The extreme ultraviolet light source apparatus according toclaim 10, wherein the prepulse laser light source is a picosecond pulselaser generating a pulse of which time width is equal to or less than apicosecond.
 15. The extreme ultraviolet light source apparatus accordingto claim 1, wherein the prepulse laser light source emits the prepulselaser light with such intensity that lets ablation occur in a part ofthe target.
 16. The extreme ultraviolet light source apparatus accordingto claim 15, wherein an irradiation intensity of the prepulse laserlight is equal to or greater than 10⁷ W/cm² but not exceeding 10⁹ W/cm².17. The extreme ultraviolet light source apparatus according to claim15, wherein the main pulse laser light source irradiates only thepre-plasma with the main pulse laser light.
 18. The extreme ultravioletlight source apparatus according to claim 15, wherein the prepulse laserlight source is a picosecond pulse laser generating a pulse of whichtime width is equal to or less than a picosecond.
 19. A method ofgenerating an extreme ultraviolet light by irradiating a target with amain pulse laser light after irradiating the target with a prepulselaser light, comprising the steps of: a pre-plasma generating step wherea pre-plasma is generated by irradiating the target with the prepulselaser light while a part of the target remains, the pre-plasma beinggenerated at a different region from a target region, the differentregion being located on an incident side of the prepulse laser light;and an extreme ultraviolet light generating step where the extremeultraviolet light is generated by irradiating the pre-plasma with themain pulse laser light.